6_Mechanical Properties.pdf - Chapter 6 Mechanical Properties of Materials INTRODUCTION Many materials when in service are subjected to forces or loads

6_Mechanical Properties.pdf - Chapter 6 Mechanical...

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Chapter 6 Mechanical Properties of Materials INTRODUCTION Many materials, when in service, are subjected to forces or loads, it is necessary to know the characteristics of the material and to design the member from which it is made such that any resulting deformation will not be excessive and fracture will not occur. The mechanical behavior of a material reflects the relationship between its response or deformation to an applied load or force. Important mechanical properties are strength, hardness, ductility, and stiffness. It is possible for the load to be tensile, compressive, or shear, and its magnitude may be constant with time, or it may fluctuate continuously. Application time may be only a fraction of a second, or it may extend over a period of many years. Service temperature may be an important factor. The role of structural engineers is to determine stresses and stress distributions within members that are subjected to well-defined loads. This may be accomplished by experimental testing techniques and/or by theoretical and mathematical stress analyses. These topics are treated in traditional stress analysis and strength of materials texts. Materials are frequently chosen for structural applications because they have desirable combinations of mechanical characteristics. Mechanical Properties of a material describes the response of the material to an applied force or torque. 1
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Session 19: Definitions of mechanical properties: Elasticity, Plasticity - atomic view, Strength, Stiffness, Hardness, Toughness, Ductility, Brittleness, Fatigue, Toughness and Fracture, Moduli of elasticity, Poisson's Ratio (Board Teaching) Stress :- The internally developed forces per unit area of a material due to the application of external force. σ= F A o Units – N/m 2 Three types of Stress 1. T ensile stress: It occurs when equal and opposite forces are directed away from each other. σ= F A o 2. Compressive stress: It occurs when equal and opposite forces are directed toward each other. 3. Shearing Stress: Force per unit area applied parallel to the surface of a body trying to displace the upper layers of the body . Shearing Stress or Tangential Stress = Force/Area Strain: - Fractional change in the dimensions of a material due to the application of external force. It has no unit. Three types of Strain: 1. Longitudinal Strain :-It is defined as the increase in length (Δl) per unit original length (l) when deformed by the external force. Longitudinal Strain= Δl l 2. Volumetric Strain :-It is defined as change in volume (ΔV) per unit original volume (v), when deformed by external force. Volume Strain= ΔV V 2
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3. Shear strain: It is defined as the tangent of the strain angle, θ Shear Strain = Tan θ. Hookes Law: Within the proportional limit, the strain produced in a body is directly proportional to the applied stress Stress α Strain, i. e., Stress = (E) Strain.
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